Turn the normal channel of you old Fender into a Plexi

Scott_F

Scott_F

Flushologist
Staff member
This one is so freaking easy and totally reversible.... I can't take credit for it, but it works.

On the normal channel of my Pro Reverb, there is a 6800k resistor connected from one wiper on the bass pot to ground. Just snip it on one end, then move it away out of the circuit and your normal channel becomes a screaming gain channel.

I'm going to go back and run a ground line in place of that resistor when I get back in town. What I did worked, but I think it can get better.

With and ABY switch, I now have a nice lead gain channel. Try it. It works.
 
Re: Turn the normal channel of you old Fender into a Plexi

Wierd.... I wish I had a fender tube amp now, so I can make it sound like a marshall... :confused:

actually, I really want a Fender tube amp.... and a JTM45 Plexi.... and a mesa single recto.... I LOVE AMPS!
 
Re: Turn the normal channel of you old Fender into a Plexi

Hmmmm :33: I just checked the schematic for my Princeton. It has the
same 6800k resistor on the bass pot. I'm gett'in all kinds of crazy Idea's. :smack:
I think I'm gonna leave it alone. But I guess I could hard wire a switch
between the resitor and ground :13: ?? What do you think the cons
are to this Mod? It seems like your sending a whole bunch of voltage
to some where it's not ment to go?
 
Re: Turn the normal channel of you old Fender into a Plexi

I always wondered why these tonal improvements never dawned on the original Fender engineers, but then again, they designed things to be a certain way, then the assembly workers did what they were supposed to do.
 
Re: Turn the normal channel of you old Fender into a Plexi

kmcguitars said:
Hmmmm :33: I just checked the schematic for my Princeton. It has the
same 6800k resistor on the bass pot. I'm gett'in all kinds of crazy Idea's. :smack:
I think I'm gonna leave it alone. But I guess I could hard wire a switch
between the resitor and ground :13: ?? What do you think the cons
are to this Mod? It seems like your sending a whole bunch of voltage
to some where it's not ment to go?
Click to expand...

Voltage going where it's not ... Nah man, what you've done ... err, what Scott did is this ... that resistor takes the place of a mid pot, it basically controls the amount of mid range scoop/dip that that channel has, I've said this before ... the Fender tone stacks are design around giving a player bass, and treble, and not much else ...even with 25K or 50K mid controls turned all the way up, (and most Fenders were 10k ~15k) compared to the 250k (or more) controls of the bass and treble, that's still a lot of mid cut (and gain loss) ... By cutting that resistor out you've reduced the insertion loss of that tone stack big time (although you've also reduced the effectiveness of those controls) ... unless I miss my guess the bass control now acts as a sort of bass trim, and the treble control is a balance between the two (as all Fenders are). If you really want to add some mids you could take that 120pF cap (bright switch) and run the wiper end to ground for a high cut (although you probably want to trim it some), or you could run the wiper end of the cap to the slope resistor side of the 250pF treble cap for added upper mids and treble, and extra brightness as you turn down the treble volume control (kinda a presence boost even though you are turning down the treble ... hard to explain). The bright *switch* itself could be used to select between the 0dB input (input 1) and the -6db (input 2) padded input, as a type of rhythm overdrive, and lead overdrive.
Think about it Scott, and it's all reverseable also ... :)
 
Re: Turn the normal channel of you old Fender into a Plexi

ooo ooo more ideas, more fun.
 
Re: Turn the normal channel of you old Fender into a Plexi

when I get my 3 prong cord, I'm also going to find something to do with that 2-way ground switch. I'm probably going to play with the negative feedback loop somehow using that switch. Just gotta think it through a bit. What sucks is that I"m going to be gone for 2 weeks straight and I'm going to have a bunch of ideas of stuff to try and won't have access to my amp.

Gotta say, even with the crappy stock speakers, that amp sounds heavenly in clean mode. The speakers can't handle the drive very well without farting out. I've got two of the new Emminence Texas Heats heading my way for this amp.

I ran the KT66HP's in there for a while last night and didn't think the amp sounded right with that tube. Went back to the RCA 6L6GC's.
 
Re: Turn the normal channel of you old Fender into a Plexi

I'd be interested to hear how those new GT 6L6's would sound in that amp Scott, combined with the new speakers.
 
Re: Turn the normal channel of you old Fender into a Plexi

most of the BF amps sound fine when you crank 'em

the mod that usually interests me is modifying the feedback loop, because that was one area that changed from the tweed circuits, in order to make the amps more "stable", but they also feel stiffer due to that change. What I like about the tweed circuits is that they're real touch sensitive.
 
Re: Turn the normal channel of you old Fender into a Plexi

Ah...if only it was that simple! It's not however.

There's alot more to making that normal channel sound like a Plexi than clipping the 6.8K midrange resistor.

And without a cathode follower circuit, you won't get the crunch of a plexi.

If you really want to improve the rock tone of the normal channel, do this:

1. Put reverb on both channels...that'll boost the gain of the normal channel.

2. Change the .1 and .047 tone caps to a pair of .02 Mallory 150's...like a Pexi Marshall or 59 tweed Bassman.

3. Change the 250 pf. treble disc cap to 250 pf silver mica...like a 59 tweed Bassman.

4. Change the 6.8K midrange resistor (the one you clipped) to 10K or even higher to add mids...or replace it with a 25K trim pot inside the amp and dial in the mids you want. When you find the value you like, measure the trim pot and then replace it with a resistor of that value.

5. Change the .047 coupling cap on the output of the 12ax7 tube of the normal channel to a .02...you don't need .047 now that the channel has more gain from mod #1.

6. Double the size of the negative feedback resistor...or triple it if you like where that takes the tone.

7. To push the front end more easily and increase sensitiity, replace the 68K input resistors (where your guitar plugs in) with a pair of 33K...works great with single coils. You could use the normal channel for Strat pickups and the other channel for humbuckers.

8. You might want to replace the 1 meg grid load resistor on the input of the amp with 2 meg...that'll give it more oomph and overdrive more easitly too.

9. You might replace the cap on the input of the phase invertor tube with a larger value too...maybe .01 or .02. That'll get a little more grind out of the amp. Use quality Orange Drops, Mallorys instead of the cheapie disc caps scattered throughtout the amp.

There's lots more that can be done...you can also replace the mixing R/C circuit that's at the end of the reverb recovery circuit to get more gain out of it...experiment.

You still won't have a Marshall tho...

Lew
 
Re: Turn the normal channel of you old Fender into a Plexi

on the first triode stage you could even ground the cathode, bypassing both the cathode resistor and cap (if one is present), and install a 5 to 10 meg grid resistor, with a cap b4 the grid stopper (the 68K). the reason for the cap is to prevent DC from getting back to the guitar.
chekc out the old fender champ or the gibson lespaul jr amp. both have contact bias (what i call it). does have a different sound.

scott, what you could do with that switch, is install a jack for a footswitch. stick a relay acorss that 6.8k, and have yourself footswitchable boost/lead.

germ
 
Re: Turn the normal channel of you old Fender into a Plexi

Two more suggestions Scott:
1. Study the chapters in Gerald Webers two books, especially the chapters on improving blackface Super Reverbs...

2. Buy the Dan Torres kit for heating up the normal channel of a Fender...it's very basic and just resistors and caps but it'll get you really familiar with what all of these parts actually do.


Lew
 
Re: Turn the normal channel of you old Fender into a Plexi

Scott, sometime soon (next couple of weeks) I may change the negative feedback thing on mine; I'll let you know how it comes out.
 
Re: Turn the normal channel of you old Fender into a Plexi

You can also shift the tone controls a bit by adjusting the slope resistor value. Typical values are between 47k and 100k. Kent had a good suggestion a while back that involved installing a 100k linear pot to control the slope resistor value. A lower value will shift things more towards the bass end for a warmer tone, a higher value will shift it more towards the treble end for more brightness. I'm thinking of installing something like this on my Classic 50 next time I take it apart.

Ryan
 
Re: Turn the normal channel of you old Fender into a Plexi

Thanks fellas! I can't wait to get back home. My first thing will be the negative feedback thing. I'm thinking of using that ground switch to engage the "normal" value, then when I flip it, I go to a larger cap to provide less NFB.

Lew, great suggestions buddy. YOu know I'm a newbie on this stuff and I love the mods!

Whatever I do, I'm taking pictures so I can put it back.


LEW! Why did they put that cap on my reverb jack? Was that a fender thing or do you think some tech did it while back?
 
Re: Turn the normal channel of you old Fender into a Plexi

Scott_F said:
Thanks fellas! I can't wait to get back home. My first thing will be the negative feedback thing. I'm thinking of using that ground switch to engage the "normal" value, then when I flip it, I go to a larger cap to provide less NFB.

Lew, great suggestions buddy. YOu know I'm a newbie on this stuff and I love the mods!

Whatever I do, I'm taking pictures so I can put it back.


LEW! Why did they put that cap on my reverb jack? Was that a fender thing or do you think some tech did it while back?
Click to expand...

Thanks Scott, I'm just a hack compared to my brother.

That must be a silverface Fender. The caps are to shunt high frequencies to ground and eliminate oscillation that could have been avoided with neater wiring.

That's one thing about those silverface Fenders...the wiring gets pretty sloppy and some use alot of excess wire, mostly from the preamp tubes to the circuit board.

I usually clip those caps off and remove them...but if you notice oscillation, you either need to correct the sloppy/excess wiring issues or put the caps back on again.

Gerald Weber says he's removed a foot or two of unshielded wire from the preamps of silverface Fenders, just by unsoldering one end and clipping off the excess and then resoldering. Just all the slop running from the preamp and reverb tubes back and forth to the circuit board...especially to the grid/input of the preamp tubes.

I've done it, and it does seem to tighten up the tone and quiet things down.

Did you do a blackface mod to the bias circuit yet? You should if the bias adjustment is currently a bias balance control rather than actually allowing th bias voltage to be adjusted.

I found the Gerald Weber books, the Dave Funk book and even Dan Torre's book, Inside Tube Amps, to all be a big help in getting a handle on this stuff and coming up with a personal understanding of how to picture how everything works.

I'm a real intuitive "tech"...I don't really understand electronic theory. But I know what all the parts do and how changing them one way or the other affects the tone.

Lew
 
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Re: Turn the normal channel of you old Fender into a Plexi

haha ... I've read all the amp books, except Torres. I just don't quite have Scott's "can do" attitude. :sad:

I've read critical comments about Gerald Weber's books, saying he was just wrong electronically about some of his ideas. But, you know what? I found his book one of the easier to understand, and it seems more common sense to me _ especially the section on "how to make your tube amp sound better". :)

I'm still not mod crazy, though. I'd say get those BF amps running well, take care of any noise or filtering problems, replace any worn tubes, bias correctly, get the right speaker in there, and play it for a while first.
 
Re: Turn the normal channel of you old Fender into a Plexi

would you guys buy a blackface 66 bassman for $200....... AB165 circuit.....??? ive got a twin but im thinkin the bassman might be cool for some warmer jazz stuff....
 
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